1. Field of the Invention
The invention relates to polyvinylidene fluoride microporous membranes and methods for their production. In particular, the invention relates to microporous polyvinylidene fluoride membranes that differ significantly in both their structural and functional characteristics from conventional polyvinylidene fluoride microporous membranes.
2. Description of the Prior Art
Polyvinylidene fluoride microporous membranes, generally formed as thin sheets of substantially uniform thickness, have a sponge-like internal structure containing millions of channels. These channels define a tortuous flow path for liquids from one side of the membrane sheet to the other side. Conventional methods of producing these polyvinylidene fluoride (hereinafter "PVDF") membranes result in membranes having a matrix of intercommunicating channels, the channels having a substantially uniform width within narrow limits.
Microporous membranes act as screens or sieves and retain on their surface all particles larger than the given width of a channel (i.e. pore diameter). Entrapment on the membrane surface of particles of approximately the pore diameter will rapidly plug the membrane irreversibly, leading to rapid decline in flow rate. Due to the tortuous nature of the flow channels in conventional microporous membranes, significant hydraulic pressure is needed to force liquids from one side of the membrane to the other. As the membranes clog, this pressure necessarily increases.
Conventionally-produced PVDF membranes are commercially available with average pore sizes (i.e. pore diameters) in the range from about 0.10 microns to about 5.0 microns. The smallest of these conventional pore sizes will retain some large viruses and most bacteria However, most viruses and some bacteria are not retained. In addition, the smallest of these conventional pore sizes will not retain large macromolecules. Attempts to produce microporous membrane filters having pore sizes less than about 0.10 micron have generally led to problems of very slow flow because of the small pore size, and to problems of rapid plugging.
A conventionally-produced PVDF membrane is disclosed by Mahoney, in U.S. Pat. No. 5,013,339. The reference discloses a composition used in preparing microporous PVDF polymer membranes. The average pore size of the microporous membranes disclosed in the reference is from about 0.05 micron to about 10.0 microns. The reference further discloses that the membranes produced are used for liquid separation processes such as microfiltration, ultrafiltration, dialysis, and membrane stripping. It is noted that ultrafiltration and microfiltration are pressure driven processes using porous membranes in which particles or solutes are separated from solutions. The reference notes that these membranes may be characterized by their hydraulic permeability and sieving coefficient. The reference defines hydraulic permeability as a flow rate, such as gallons/ft..sup.2 /day (GFD) at a given pressure. More specifically, hydraulic permeability is defined as the volume of a solvent transported through the membrane under the influence of a pressure gradient. The membranes disclosed in the Mahoney reference preferably have a hydraulic permeability for water, at 25.degree. C., of at least about 10.0 ml/m.sup.2 /hr/cmHg.
Conventional solvent-casting procedures for producing microporous PVDF membranes rely on the use of a solvent such as acetone for the PVDF polymer. Nevertheless, acetone is usually not thought of as a solvent for this particular polymer because it is exceedingly difficult to dissolve any appreciable quantity of PVDF in acetone at room temperature. In order to dissolve a sufficient quantity to form an adequately viscous solution for use in practicing conventional methods, the acetone must be heated close to its boiling point of about 50.degree. C. This produces severe constraints on conventional methods since the initial mixing of PVDF must occur at an elevated temperature.